Single control valve for vacuum system

ABSTRACT

A single control valve for a vacuum system having an evacuable chamber, a mechanical pump and a diffusion pump all of which are capable of being connected to means for supplying air. This valve comprises a valve body and an enclosure inside the valve body. The enclosure is formed of a first, a second, a third and a fourth chamber, all these chambers being in consecutive order in the continuation of one another and capable of communication with one another. A first duct connects the first chamber to the diffusion pump and a second duct connects the first chamber to the chamber to be evacuated. A third duct connects the second chamber to the mechanical pump, a fourth duct connects the third chamber to the diffusion pump and a fifth duct connects the fourth chamber to the entry of air. Means are provided to create a vacuum in the chamber to be evacuated in a four step cycle. In the first step, the diffusion pump is isolated from the chamber to be evacuated by blocking the first duct and closing all communication between the second and the third chamber while allowing the mechanical pump to communicate with the chamber to be evacuated via the second and first chambers and the respective ducts. In the second step, the mechanical pump is isolated from the chamber to be evacuated and the diffusion pump is kept isolated in order that only the first chamber is in communication with the chamber to be evacuated. In the third step, the chamber to be evacuated is kept isolated from the mechanical pup and from the diffusion pump and the mechanical pump is allowed to pump in the diffusion pump by opening the communication between the second chamber, the third chamber, the fourth duct and the diffusion pump. Finally, in the fourth step the operating conditions of the third step are maintained except that the first duct is opened to prevent any entry of air via the fifth duct with the result that the diffusion pump communicates with the chamber to be evacuated via the first chamber thus contributing to establish a high vacuum in the system to be evacuated. Control means are provided to carry out the four steps aforementioned.

United States Patent 1 Picker 1 Jan. 30, 1973 1 SINGLE CONTROL VALVE FOR VACUUM SYSTEM [75] Inventor: Patrick Picker, Sherbrooke, Canada [73] Assignee: Universite De Sherbrooke, Sherbrooke, Quebec, Canada [22] Filed: Feb. 16, 1971 [21] Appl. No.: 115,544

[52] U.S. Cl ..137/566,137/624.18 [51] Int. Cl ..F16k 11/00 [58] Field of Search ..137/566, 567, 624.18

[56] References Cited UNITED STATES PATENTS 2,945,445 7/1960 Smith et al ..137/567 X 3,164,170 1/1965 Gutter ..l37/624.l8 X 3,307,577 3/1967 Dorwald et al. ..137/S67 Primary Examiner-William R. Cline AztorneyCushman, Darby & Cushman [57] ABSTRACT A single control valve for a vacuum system having an evacuable chamber, a mechanical pump and a diffusion pump all of which are capable of being connected to means for supplying air. This valve comprises a valve body and an enclosure inside the valve body. The enclosure is formed of a first, a second, a third and a fourth chamber, all these chambers being in consecutive order in the continuation of one another and capable of communication with one another. A

first duct connects the first chamber to the diffusion pump and a second duct connects the first chamber to the chamber to be evacuated. A third duct connects the second chamber to the mechanical pump, a fourth duct connects the third chamber to the diffusion pump and a fifth duct connects the fourth chamber to the entry of air. Means are provided to create a vacuum in the chamber to be evacuated in a four step cycle. In the first step, the diffusion pump is isolated from the chamber to be evacuated by blocking the first duct and closing all communication between the second and the third chamber while allowing the mechanical pump to communicate with the chamber to be evacuated via the second and first chambers and the respective ducts. in the second step, the mechanical pump is isolated from the chamber to be evacuated and the diffusion pump is kept isolated in order that only the first chamber is in communication with the chamber to be evacuated. in the third step, the chamber to be evacuated is kept isolated from the mechanical pup and from the diffusion pump and the mechanical pump is allowed to pump in the diffusion pump by opening the communication between the second chamber, the third chamber; the fourth duct and the diffusion pump. Finally, in the fourth step the operating conditions of the third step are maintained except that the first duct is opened to prevent any entry of air via the fifth duct with the result that the diffusion pump communicates with the chamber to be evacuated via the first chamber thus contributing to establish a high vacuum in the system to be evacuated. Control means are provided to carry out the four steps aforementioned.

9 Claims, 6 Drawing Figures PATENIEDJMO ma 3.713.460

SHEET 1 OF 3 76 1 INVENTOR Patri I PICKER ORNEY PATENTEDJAN30 I973 3.713.460

SHEET 2 0F 3 INVENTOR Patrick PICKER 6 ATTORNEY SINGLE CONTROL VALVE FOR VACUUM SYSTEM The present invention is directed to a single control valve for a vacuum system. A vacuum chamber is normally formed of a system to be evacuated, a primary mechanical pump and a diffu- I limeter of mercury. If a higher vacuum is desired, the

common practice is to thereafter use a diffusion pump. Since the diffusion pump is used only in the last stage of the process, it would usually only be operated if there already existed a satisfactory vacuum inside the chamber.

The classical arrangement usually requires three valves; this logically follows from what has just been said. The chamber is first evacuated by the mechanical pump, then by the mechanical pump and the diffusion pump combined. if there are two pumps and one of them can operate only above a certain pressure, this immediately poses a problem. The chamber must first of all be evacuated by means of the mechanical pump without the use of the diffusion pump. Consequently, there must be a combination of valves which will permit this operation. By making the diagram of the various possibilities one would logically arrive at a minimum of three valves. The system is therefore evacuated in the first place by the mechanical pump and then by the mechanical and diffusion pumps combined by using the three valves.

Further, the order according to which these three valves must be opened and closed, should be strictly observed. As an example it will be assumed that the diffusion pump is already in operation. There is a valve at the inlet and a valve at the outlet and both valves are closed. These are essential conditions since if there is an atmospheric pressure in the system the oil of the diffusion pump would be burnt. To create a vacuum in the chamber the first operation consists of opening the valve which connects the mechanical pump to the chamber to be evacuated. However, the valve which is at the outlet of the diffusion pump and the one which is at the inlet should not be opened, since air would be allowed to penetrate inside the diffusion pump. It will therefore be seen that there is an order of opening of the valves which must strictly be followed. In a system to be evacuated, these operations are not so obvious since all these valves look alike. It will therefore be realized that a simple manual mistake in the order of opening of the valves is enough to burn the diffusion pump.

Another noted disadvantage is the necessity of simultaneously manipulating these three valves. As result, the operation is complicated and boring and can also cause certain damages as aforementioned.

The invention described hereinbelow, which has been conceived for the purpose of simplification combines these three valves into a single control which permits safely carrying out the normal cycle of operations which are required to create a vacuum.

Broadly speaking, the invention relates to a single control valve for a vacuum system having an evacuable chamber, a mechanical pump and a diffusion pump all of which are capable of being connected to an air supply. This valve comprises a valve body and an enclosure inside the valve body. The enclosure is formed ofa first, a second, a third and a fourth chamber, all these chambers being in consecutive order in the continuation of one another and capable of communication with one another. A first duct connects the first chamber to the diffusionpump and a second duct connects the first chamber to the chamber to be evacuated. A third duct connects the second chamber to the mechanical pump, a fourth duct connects the third chamber to the diffusion pump and a fifth duct connects the fourth chamber to the entry of air. Means are provided to create a vacuum in the chamber to be evacuated in a four step cycle. In the first step, the diffusion pump is isolated from the chamber to be evacuated by blocking the first duct and closing all communication between the second and the third chamber while allowing the mechanical pump to communicate with the chamber to be evacuated via the second and first chambers and the respective ducts. in the second step, the mechanical pump is isolated from the chamber to be evacuated and the diffusion pump is kept isolated in order that only the first chamber is in communication with the chamber to be evacuated. in the third step, the system to be evacuated is kept isolated from the mechanical pump and from the diffusion pump and the mechanical pump is allowed to pump in the diffusion pump by opening the communication between the second chamber, the third chamber, the fourth duct and the diffusion pump. Finally in the fourth step the operating conditions of the third step are maintained except that the first duct is opened to prevent any entry of air via the fifth duct with the result that the diffusion pump communicates with the chamber to be evacuated via the first chamber thus contributing to establish a high vacuum in the chamber to be evacuated. Means are provided to control the four steps aforementioned.

In accordance with a preferred embodiment of the invention, a trap valve is provided in the first chamber at the entry into the first duct in order to close or open at will the communication of the latter with the diffusion pump.

The trap valve should preferably be provided with a spring arrangement which keeps the valve closed at the entry into the first duct during the first, second and third steps.

In accordance with another embodiment of the invention, a valve allowing an entry of air is mounted along the fifth duct to permit the air to penetrate in the chamber to be evacuated, if necessary, during the second and third steps.

The means used to create a vacuum inside the chamber to be evacuated can optionally comprise inter alia a slide valve cooperating with the trap valve and mounted within the enclosure. The slide valve has a first and a second extremity. These extremities have the same cross-section which is identical to the cross-section of the second chamber but is lesser than the crosssections of the first and third chambers. The slide-valve is movable between four different positions in accordance with the four steps aforementioned so that in the first step, the first extremity is located in the first chamber and the second extremity is in the second chamber past the third duct, in the second step the first extremity closes the connection between the first and the second chamber, while the second extremity closes all connection between the second and third chambers, during the third step the first extremity is inside the second chamber ahead of the third duct and the second extremity is inside the third chamber, and finally in the fourth step the slide valve moves again to displace the trap valve from the position where it is closing all connection between the first chamber and the diffusion pump while the first extremity is still in the second chamber ahead of the third duct and the second extremity is in the third chamber.

In accordance with the invention there can be provided a hollow rod in the body of the valve inside the enclosure, one of the ends of the hollow rod being formed by the first extremity, a collar being provided on the rod to define the second extremity of the slide valve, the other end of the rod being connected to the control means, it being understood that the rod must move in accordance with the aforesaid movements of the slide valve.

The end of the hollow rod formed by the first extremity of the slide valve is provided with an opening. A solid rod is introduced in this opening and is terminated by a flange at the end thereof which is located inside the hollow rod. At the end of the solid rod which is outside the hollow rod, there is a plate which is used to close the entry of the first duct in the diffusion pump. A first spring surrounds the solid rod between the flange and the first extremity of the slide valve, and a second spring surrounds the solid rod between the first extremity ofthe slide valve and the plate in order to define the trap valve used to open or close the first duct.

The hollow rod may be provided with an opening at a level corresponding to the fifth duct when the rod is in the first, second and third step.

In accordance with the invention, the control means can be formed with a control arm which is fixedly mounted at the other end of the hollow rod and with a zig-zag slot provided in the body of the valve in which the control arm can move between four positions corresponding to the four steps of the main rod and consequently of the slide valve.

The invention will now be described in detail, by way of an example, with reference to the accompanying drawings in which FIG. 1 is an axial longitudinal section of a single control valve according to the invention;

FIG. 2 is a perspective view of the programming control;

FIG. 3 is a schematic representation of the initial position of the valve;

FIG. 4 is a schematic representation of the first intermediate position of the valve;

FIG. 5 is a schematic representation of the second intermediate position of the valve; and

FIG. 6 is the final position of the valve during which a high vacuum starts to form in the system to be evacuated.

With reference to the drawings, it will be seen that the single control valve 1 is especially intended to be used in association with a vacuum chamber. The latter comprises a system to be evacuated (not illustrated in The valve 1 comprises a valve body 7 and an enclosure inside the valve body 7. The lower part 9 of the valve 1 rests on a seat 11 which is built to fit over the diffusion pump 3 (the latter has not been illustrated in FIG. 1).

Referring to the aforementioned enclosure of the valve 1, it will be seen that it comprises a first chamber 13, a second chamber 15, a third chamber 17 and a fourth chamber 19. These 'four chambers 13, l5, l7 and 19 are situated in order in continuations of one another and they can eventually communicate with one another. Each chamber has a different cross-section depending upon the needs and circumstances. For example, the decreasing order of the cross-sections of these chambers will be the following: 13, 17, 15 and 19, it being well understood that others values can be set if desired provided the same results are obtained.

In this vacuum chamber, as stated above, due consideration must be had to the system to be evacuated, the mechanical pump, the diffusion pump, the air entrance and obviously also the single control valve. To connect these various parts together, there will be used a plurality of ducts which will now be described.

The diffusion pump 3 is connected to the chamber 13 via a duct 21; a duct 23 connects this chamber 13 to the chamber to be evacuated (not illustrated in the drawings). With respect to chamber 15, it is connected to the mechanical pump (not illustrated in the drawings) by means of the duct 25. The chamber 17 is also connected to the diffusion pump, rearwardly thereof, via duct 27 of which only the entry in chamber 17 is shown in FIG. 1. Finally there is a fifth duct 29 which is used to introduce air in chamber 19. It should be observed that an air inlet valve 31 (identified V in FIGS. 3 to 6) is provided along duct 29 to enable the air to be introduced, if necessary, in the chamber to be evacuated during the second and third steps, as will be explained hereinafter.

The portion 33 where the air is admitted in the valve 1 is preferably made of a separate element which is adjusted at the uppermost portion 35 of the valve body. Finally to surmount this portion 33 there is a skirt 37 in which the selection of the programming control is carried out. Again this will be discussed more in detail later on.

A hollow rod 39 is inserted into the body of the valve 7 inside the enclosure in order to eventually be present in the four chambers 13, l5, 17, 19 as well as inside the skirt 37. The two extreme lower and upper positions are clearly illustrated in FIGS. 3 and 6 of the drawings. The lower extremity of the rod 39 comprises a piston 41, and a second piston 43 is formed at about the upper third of the rod, these two pistons 41 and 43 defining a slide valve. Referring to the lower extremity of the rod 39 and the piston 43, it will be realized that the latter has an axial bore 45. A solid rod 47 has been introduced inside rod 39 through this opening. This solid rod 47 is terminated by a flange 49 at the end thereof which is inside the hollow rod. With respect to the lower extremity of the solid rod 47, it is terminated by a plate 51 built to close the entrance of duct 21 into chamber 13. A coil spring 53 surrounds the solid rod 47 between the flange 49 and the piston 45. Another coil spring 55 is mounted between the lower face of piston 41 and the plate 51.

Reference should continuously be made to FIG. 1 for a specific illustration of the various elements of the valve 1. It will be seen that the solid rod 41, the two springs 53, 55 and the plate 51 define a trap valve intended to open and close at will the duct 21 in accordance with the orders given by the program. When the two pistons of the slide valve are simultaneously in chamber 15, they must prevent any leak or entry of air. This is the reason why O-rings 57 and 59 have been provided respectively on pistons 41 and 43.

In chamber 19, inside portion 33 of the single control valve 1, there is a bushing 61 to control the entry of air in the system. Above the bushing 61, there is an O-ring 63 and there is also another O-ring 65 at the lower part of the bushing 61. Another bushing 67 is disposed between O-ring 63 and O-ring 69. The hollow rod 39 is provided with an opening 71 which is in the vicinity of the duct 29 when the rod is in the first, second or third step.

Finally, to control the programming control of valve 1, the skirt 37 has a zig-zag slot 73. A control arm 75 provided with a handle 77 is fixedly mounted in the upper part of the hollow rod 39. By reference to FIG. 2 it will be seen that the latter can be moved between four positions.

The operation of the single control valve will be illustrated with reference to FIGS. 3 to 6 of the drawings.

POSITION I The diffusion pump is isolated from the system to be evacuated. i

The mechanical pump is connected to the system.

Since the valve controlling the entry of air is closed, a primary vacuum is established.

This is the starting position.

POSITION II The diffusion pump is still isolated.

The mechanical pump is no longer connected with the chamber to be evacuated.

In this intermediate position, manipulations of short duration can be carried out on the mechanical pump.

POSITION III POSITION IV The diffusion pump is connected with the chamber to be evacuated. A high vacuum starts to form.

The mechanical pump still operates rearwardly of the diffusion pump. An entry of air is then absolutely impossible (except if there is a sudden and accidental increase of the pressure in the system) because of the respective positions of the orifice 71 and the valve 31 which allows the air to come in.

In the position IV, a return to positions III or II would keep the system under a high vacuum.

During the passage from position III to position IV, if the primary vacuum inside the chamber to be evacuated is not sufficient, there will be no communication between the chamber and the diffusion pump, since the pressure inside the chamber will keep the stop-valve of the diffusion pump in the closed position.

Iclaim:

1. A single control valve for a vacuum system having an evacuable chamber, a mechanical pump and a diffusion pump all of which can be connected to means for supplying air, said valve comprising:

a valve body and an enclosure inside said valve body, said enclosure formed of a first, a second, a third and a fourth chamber, all these chambers being in consecutive order in the continuation of one another and in communication with one another,

a first duct connecting the first chamber to the diffusion pump, a second duct connecting the first chamber to said evacuable chamber, a third duct connecting the second chamber to the mechanical pump, a fourth duct connecting the third chamber to the diffusion pump and a fifth duct connecting the fourth chamber to said air supplying means,

means to create a vacuum in said evacuable chamber in a four step cycle wherein during the first step, the diffusion pump is isolated from the evacuable chamber by blocking the first duct and closing all communication between the second and the third chamber while allowing the mechanical pump to communicate with the evacuable chamber via the second and the first chambers and the respective ducts, during the second step, the mechanical pump is isolated from the evacuable chamber and the diffusion pump is kept isolated in order that only the first chamber be in communication with the evacuable chamber, during the third step the evacuable chamber is kept isolated from the mechanical pump and from the diffusion pump and the mechanical pump is allowed to pump in the diffusion pump by opening the communication between the second chamber, the third chamber, the fourth duct and the diffusion pump, and finally during the fourth step the operating conditions of the third step are maintained except that the first duct is opened to prevent any entry of air via the fifth duct with the result that the diffusion pump communicates with said evacuable chamber via the first chamber thus contributing to the establishment of a high vacuum in the evacuable chamber, and

control means for carrying out the four steps aforementioned.

2. A single control valve according to claim 1, wherein a trap valve is provided in the first chamber at the entry into the first duct in order to close or open at will communication of the latter with the diffusion pump.

3. A single control valve according to claim 2, wherein the trap valve is provided with a spring arrangement which keeps the valve closed at the said entry into said first duct during the first, second and third steps.

4. A single control valve according to claim 3, wherein a slide valve cooperating with the trap valve is provided inside the enclosure, said slide valve having a first and a second extremity, these extremities having the same cross-section, said cross-section identical to the cross-section of the second chamber but lesser than the cross-sections of the first and third chambers, said slide valve movable between four different positions in accordance with the four steps aforementioned so that during the first step, the first extremity is located in the first chamber and the second extremity is in the second chamber past the third duct, during the second step, the first extremity closes the connection between the first and the second chamber, while the second extremity closes all connection between the second and third chambers, during the third step the first extremity is inside the second chamber ahead of the first duct and the second extremity is inside the third chamber, and finally during the fourth step the slide valve moves again to displace the trap valve from the position where it closes all connection between the first chamber and the diffusion pump while the first extremity is still in the second chamber ahead of the third duct and the second extremity is in the third chamber.

5. A single control valve according to claim 4, wherein a hollow rod is inserted in the body of the valve inside the enclosure, one of the ends of the hollow rod being formed by the first extremity, a collar is provided on the rod to define the second extremity of the slide valve, the other end of the rod being connecting to the control means so, that the rod must move in accordance with the aforementioned movements of the slide valve.

6. A single control valve according to claim 5, wherein the end of the hollow rod formed by the first extremity of the slide valve is provided with an opening, a solid rod is introduced in said opening and is terminated by a flange at the end thereof which is located inside said hollow rod, the end of said solid rod which is outside the hollow rod is formed with a plate which is used to close the entry of the first duct in the diffusion pump, a first spring surrounding the solid rod between the flange and the first extremity of the slide valve, and a second spring surrounding the solid rod between the first extremity of the slide valve and the plate in order to define the trap valve used to open or close the first duct.

7. A single control valve according to claim 6, which comprises at least one opening in said hollow rod at the level corresponding to the fifth duct when the hollow rod is in the first, second and third step.

8. A single control valve according to claim 7,

wherein the control means are formed with a control arm which is fixedly mounted at the other end of the hollow rod and with a zig-zag slot provided in the body of the valve in which the control arm can move between four positions corresponding to the four steps of the main rod and consequently of the slide valve.

9. A single control valve according to claim 1, wherein a valve allowing an entry of air is mounted along the fifth duct to permit the air to penetrate in the evacuable chamber, if necessary, during the second and third step. 

1. A single control valve for a vacuum system having an evacuable chamber, a mechanical pump and a diffusion pump all of which can be connected to means for supplying air, said valve comprising: a valve body and an enclosure inside said valve body, said enclosure formed of a first, a second, a third and a fourth chamber, all these chambers being in consecutive order in the continuation of one another and in communication with one another, a first duct connecting the first chamber to the diffusion pump, a second duct connecting the first chamber to said evacuable chamber, a third duct connecting the second chamber to the mechanical pump, a fourth duct connecting the third chamber to the diffusion pump and a fifth duct connecting the fourth chamber to said air supplying means, means to create a vacuuM in said evacuable chamber in a four step cycle wherein during the first step, the diffusion pump is isolated from the evacuable chamber by blocking the first duct and closing all communication between the second and the third chamber while allowing the mechanical pump to communicate with the evacuable chamber via the second and the first chambers and the respective ducts, during the second step, the mechanical pump is isolated from the evacuable chamber and the diffusion pump is kept isolated in order that only the first chamber be in communication with the evacuable chamber, during the third step the evacuable chamber is kept isolated from the mechanical pump and from the diffusion pump and the mechanical pump is allowed to pump in the diffusion pump by opening the communication between the second chamber, the third chamber, the fourth duct and the diffusion pump, and finally during the fourth step the operating conditions of the third step are maintained except that the first duct is opened to prevent any entry of air via the fifth duct with the result that the diffusion pump communicates with said evacuable chamber via the first chamber thus contributing to the establishment of a high vacuum in the evacuable chamber, and control means for carrying out the four steps aforementioned.
 1. A single control valve for a vacuum system having an evacuable chamber, a mechanical pump and a diffusion pump all of which can be connected to means for supplying air, said valve comprising: a valve body and an enclosure inside said valve body, said enclosure formed of a first, a second, a third and a fourth chamber, all these chambers being in consecutive order in the continuation of one another and in communication with one another, a first duct connecting the first chamber to the diffusion pump, a second duct connecting the first chamber to said evacuable chamber, a third duct connecting the second chamber to the mechanical pump, a fourth duct connecting the third chamber to the diffusion pump and a fifth duct connecting the fourth chamber to said air supplying means, means to create a vacuuM in said evacuable chamber in a four step cycle wherein during the first step, the diffusion pump is isolated from the evacuable chamber by blocking the first duct and closing all communication between the second and the third chamber while allowing the mechanical pump to communicate with the evacuable chamber via the second and the first chambers and the respective ducts, during the second step, the mechanical pump is isolated from the evacuable chamber and the diffusion pump is kept isolated in order that only the first chamber be in communication with the evacuable chamber, during the third step the evacuable chamber is kept isolated from the mechanical pump and from the diffusion pump and the mechanical pump is allowed to pump in the diffusion pump by opening the communication between the second chamber, the third chamber, the fourth duct and the diffusion pump, and finally during the fourth step the operating conditions of the third step are maintained except that the first duct is opened to prevent any entry of air via the fifth duct with the result that the diffusion pump communicates with said evacuable chamber via the first chamber thus contributing to the establishment of a high vacuum in the evacuable chamber, and control means for carrying out the four steps aforementioned.
 2. A single control valve according to claim 1, wherein a trap valve is provided in the first chamber at the entry into the first duct in order to close or open at will communication of the latter with the diffusion pump.
 3. A single control valve according to claim 2, wherein the trap valve is provided with a spring arrangement which keeps the valve closed at the said entry into said first duct during the first, second and third steps.
 4. A single control valve according to claim 3, wherein a slide valve cooperating with the trap valve is provided inside the enclosure, said slide valve having a first and a second extremity, these extremities having the same cross-section, said cross-section identical to the cross-section of the second chamber but lesser than the cross-sections of the first and third chambers, said slide valve movable between four different positions in accordance with the four steps aforementioned so that during the first step, the first extremity is located in the first chamber and the second extremity is in the second chamber past the third duct, during the second step, the first extremity closes the connection between the first and the second chamber, while the second extremity closes all connection between the second and third chambers, during the third step the first extremity is inside the second chamber ahead of the first duct and the second extremity is inside the third chamber, and finally during the fourth step the slide valve moves again to displace the trap valve from the position where it closes all connection between the first chamber and the diffusion pump while the first extremity is still in the second chamber ahead of the third duct and the second extremity is in the third chamber.
 5. A single control valve according to claim 4, wherein a hollow rod is inserted in the body of the valve inside the enclosure, one of the ends of the hollow rod being formed by the first extremity, a collar is provided on the rod to define the second extremity of the slide valve, the other end of the rod being connecting to the control means so, that the rod must move in accordance with the aforementioned movements of the slide valve.
 6. A single control valve according to claim 5, wherein the end of the hollow rod formed by the first extremity of the slide valve is provided with an opening, a solid rod is introduced in said opening and is terminated by a flange at the end thereof which is located inside said hollow rod, the end of said solid rod which is outside the hollow rod is formed with a plate which is used to close the entry of the first duct in the diffusion pump, a first spring surrounding the solid rod between the flange and the first extremIty of the slide valve, and a second spring surrounding the solid rod between the first extremity of the slide valve and the plate in order to define the trap valve used to open or close the first duct.
 7. A single control valve according to claim 6, which comprises at least one opening in said hollow rod at the level corresponding to the fifth duct when the hollow rod is in the first, second and third step.
 8. A single control valve according to claim 7, wherein the control means are formed with a control arm which is fixedly mounted at the other end of the hollow rod and with a zig-zag slot provided in the body of the valve in which the control arm can move between four positions corresponding to the four steps of the main rod and consequently of the slide valve. 